Structure-function analyses of murine Ia molecules are being approached through the in vitro production and characterization of B hybridomas that express mutant Ia molecules. Mutant Ia-bearing cells are being produced by chemical mutagenesis and immunoselection. The resulting mutants are analyzed serologically with a panel of monoclonal anti-Ia antibodies, functionally with a panel of class II-restricted T cells, biochemically by two-dimensional gel and peptide map analyses, and by DNA sequence analysis of the mutated gene. (LB)